Substrates having one or more metal layers deposited on one or more of their surfaces have many industrial uses. For instance, insulating films having a metal layer or layers on one or more surfaces are of use in separation processes, fuel cells, super-capacitors, electrolytic cells for splitting water into hydrogen and oxygen, and for catalysis.
Electroplating (also known as electro-deposition) is a well known industrial process that uses electrical current to reduce ionic precursors of a desired material (usually a metal) from a solution (aqueous or non-aqueous) in order to coat a conductive substrate with a layer of the desired material. The electrically conductive substrate may be suitably used as one of the electrodes in an electrochemical cell whereby the electrical current causing electroplating is provided, effecting the electroplating. U.S. Pat. No. 6,203,925 (Attard et. al.) and international patent application Publication WO 99/00536 (Bartlett et al) disclose the production of porous metal films on electrically conducting substrates by electro-deposition from a liquid crystalline phase.
publication U.S. Pat. No. 2,532,283 (Brenner & Riddell) discloses electroless plating. Electroless plating is an auto-catalytic reaction used to deposit a coating of a desired material (usually a metal) from an electroless plating solution (usually an aqueous solution) onto a substrate (usually a conducting substrate). In contrast to electroplating, electroless plating does not require application of an electric current. Instead, deposition proceeds as a chemical reaction at the surface of the substrate.
Once deposition by electroless plating has been initiated, the electroless reaction is typically self-sustaining until either: a) the electroless plating solution is exhausted, b) the substrate is removed from the electroless plating solution, or c) the composition of the solution is adjusted to terminate plating. Electroplating is suitable for use with electrically conducting substrates. Electroless plating offers the advantage of allowing deposition of material by plating onto electrically insulating substrates.
Prior electroless plating methods require substantial preparation of the substrate prior to commencement of plating. Typically, prior to commencement of electroless plating, a catalytic metal such as palladium is first deposited onto the substrate as a seed or nucleation layer, most usually in a two step process, for example: i) priming of the substrate with an acidic tin chloride solution is carried out, this resulting in adsorption of Sn2+ ions onto the surface from the solution to provide a primed surface; ii) treatment of the primed surface with a solution of palladium chloride in hydrochloric acid whereby Sn2+ ions adsorbed in step (i) reduce Pd(II) ions in solution to form deposits of metallic Pd on the substrate surface.
These Pd deposits subsequently act as nucleation sites during electroless plating of the desired metal, which may be Pd or which may be a different metal. It is also known, in the prior art, to combine the metal ions for priming and those used for providing nucleation sites on the surface to be treated in a single pre-treatment solution. This may result in initial adsorption of Sn—Pd particles onto the surface of the substrate, with the tin subsequently removed by treatment with concentrated hydrochloric acid to leave palladium deposits on the surface to act as nucleation sites for electroless deposition. However, irrespective of which pre-preparation method is used to provide a substrate ready for electroless deposition, the pre-preparation method may have to be repeated up to ten times in order to achieve a surface adequately modified to act as a template for electroless deposition.
This is both costly and time consuming. Also, lengthy surface preparation, involving cleaning, etching and neutralising, may be required, prior to electroless plating and/or prior to pre-treatment. This is in order to promote adhesive bonding of the electroless metal layer to the substrate surface upon which it is deposited. Furthermore, the resulting metal deposit, generated by electroless plating, will contain Pd and may include Sn, this latter metal being a particularly troublesome and undesirable impurity if the substrate upon which the metal layer has been deposited is for subsequent use in electrochemical applications.
Photocatalytic deposition of metals onto semiconductors is a mature technology in the prior art. In particular, titanium dioxide is known as a photocatalyst for use in the reductive deposition of noble metals such as palladium and platinum onto semiconductor surfaces. During irradiation of titanium dioxide, using actinic radiation such as ultra-violet radiation having a photon energy in excess of the band gap of the titanium dioxide, electrons may be excited and drive redox reactions, catalysed by the titanium dioxide particles and leading to deposition of metal from a treatment solution onto the semiconductor surface.
However, such processes typically result in either:    a) a layer of deposited nano-particles of metal with a granular structure and highly disordered porosity, or    b) a coherent monolayer of metal, the formation of which results in cessation of the plating process once the surface and its photocatalyst layer are masked.    There is no control of layer thickness afforded in either instance.
It is also known to use photo-catalysis such as titanium dioxide in etching of surfaces as well as deposition. U.S. Pat. No. 7,067,237 and US patent application publication 2006-0019076-A disclose modification of surfaces using a photocatalyst followed by electroless deposition onto the modified surface. The processes disclosed in these documents comprise separate, discrete process steps for photoexcitation, primer metal nucleation and then electroless metal deposition.
Hence, there is a need for electroless methods of deposition of metal layers onto substrates which obviate the complex substrate treatment regimes required in the prior art and which eliminate the need for primer or nucleation metals to be present in or below deposited layers.